3 answers

I guess the root cause of the error could be some error when devstack tried to clone 'horizon'. I would suggest you try out cloning the repo manually (in '/opt/stack/horizon' directory), if it goes fine, just rerun the stack.sh script. If you face any issues cloning, resolve them and move ahead to running the stack.sh script.

If you are behind a network having Proxies, you might want to manually set up proxy for both http and https (export http_proxy=http://a.b.c.d:8080). also make sure to have sufficient sudo/root privileges while running Devstack (may be run $ sudo ./stack.sh or in root mode #./stack.sh). Hope it Helps.